Exam 4 made simple

Lecture 1: Molecular Biology Overview

Key Levels of Organization

• Molecular level: DNA, RNA, and proteins

• Sub-cellular level: Ribosomes and nucleus

• Cellular level: Cell division

Proteins

Proteins have many functions, including transporting molecules, acting as enzymes, and maintaining structure. Their shape determines their function, which is influenced by how they fold.

• Primary structure: Sequence of amino acids

• 3D structure: Polypeptide folds into a functional protein

• The sequence of amino acids is determined by the DNA sequence, a process known as gene expression. This is described by the central dogma of molecular biology:

• DNA → RNA → Protein

• Gene expression involves transcription (DNA → RNA) and translation (RNA → protein).

DNA Structure

• DNA is the genetic material for all living organisms, from bacteria to humans. It forms a double helix.

• Prokaryotes have a single circular DNA molecule.

• Eukaryotes have multiple linear chromosomes in the nucleus.

• DNA is made of nucleotides:

• Base + Sugar + Phosphate = Nucleotide

• Base groups: Pyrimidines (C, T, U) and Purines (A, G)

Key Scientist Contributions

• 1920s: Chromosomes were recognized as hereditary material composed of DNA and proteins.

• 1940s: DNA was confirmed as the genetic material.

• 1953: Francis Crick and James Watson, with the help of Rosalind Franklin’s X-ray images, discovered the structure of DNA.

• DNA is double-stranded, and the bases pair specifically (A pairs with T, and G pairs with C).

DNA Function

• Genetic information is stored in the sequence of DNA bases.

• Mutations: Changes in the base sequence can alter the genetic code.

• Replication: DNA can be copied with high precision (A-T, C-G pairing).

• Expression: DNA is used to produce proteins that influence traits.

Lecture 2: Gene Expression

Gene Expression Overview

• Genes in human cells (about 20,000) contain instructions for making proteins.

• Gene expression varies by cell type, depending on the cell’s needs.

Central Dogma of Molecular Biology

• DNA → RNA → Protein

• Transcription: Copies DNA into mRNA (same language).

• Translation: Converts mRNA into a protein sequence (different language).

Transcription Process

• RNA Types:

• mRNA: Messenger RNA that is a complementary copy of a gene.

• tRNA: Transfer RNA that brings amino acids to the ribosome.

• rRNA: Ribosomal RNA, part of the ribosome.

• Transcription involves three main stages:

• Initiation: RNA polymerase binds to the promoter region of DNA.

• Elongation: RNA polymerase synthesizes RNA in the 5’ → 3’ direction, reading the DNA 3’ → 5’.

• Termination: RNA polymerase detaches, releasing the newly synthesized RNA.

RNA Synthesis

• In RNA, Thymine (T) is replaced by Uracil (U).

• RNA is synthesized in the 5’ → 3’ direction, opposite to the DNA template strand.

Lecture 3: mRNA Processing & Translation

mRNA Processing in Eukaryotes

• Eukaryotic genes have introns (non-coding) and exons (coding).

• Introns are removed during RNA processing.

• At the 5’ end, a cap (modified guanine) is added to help mRNA bind to ribosomes.

• At the 3’ end, a poly-A tail (100-300 adenines) is added to protect mRNA and help it exit the nucleus.

Translation Overview

• The genetic code is a series of codons (3 RNA bases) that code for amino acids.

• Start codon: AUG (codes for Methionine)

• Stop codons: UAA, UAG, UGA (signals termination)

How Translation Works

• tRNA carries amino acids to the ribosome. Each tRNA has an anticodon that is complementary to the mRNA codon.

• The ribosome matches tRNA anticodons to mRNA codons and links amino acids to form a polypeptide.

Lecture 4: Detailed Translation Process

Translation Process

• Translation occurs in the 5’ → 3’ direction on mRNA.

• The ribosome has two subunits, large and small, with three binding sites for tRNA:

• A site: tRNA brings amino acids

• P site: Peptide bond formation

• E site: tRNA exits the ribosome

Steps of Translation

1. Initiation: The small ribosomal subunit binds to the mRNA’s start codon, and the tRNA carrying methionine binds. The large subunit joins.

2. Elongation: tRNA reads each codon, and amino acids are linked by peptide bonds to form a polypeptide.

3. Termination: A release factor binds to the stop codon, and the ribosome disassembles, releasing the polypeptide.

Lecture 5: Cell Division

Cell Cycle Overview

• Interphase: The cell grows and prepares for division.

• G1: Cell growth and activity

• S phase: DNA replication (sister chromatids form)

• G2: Organelles duplicate, DNA condenses

• Mitotic Phase: Nucleus divides (mitosis) and cytoplasm divides (cytokinesis).

Mitosis vs. Meiosis

• Mitosis: Creates two identical diploid daughter cells (used for growth and repair).

• Meiosis: Reduces chromosome number by half to form haploid gametes (used for reproduction).

Mitosis Phases

1. Prophase: Chromosomes become visible, and spindle fibers form.

2. Prometaphase: Nuclear envelope breaks down, and spindle fibers attach to chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate.

4. Anaphase: Sister chromatids separate and move to opposite poles.

5. Telophase: Chromosomes decondense, and the nuclear envelope reforms.

Cytokinesis

• Division of the cytoplasm occurs differently in plant and animal cells.

• In animal cells, the cell membrane pinches to form two daughter cells.

• In plant cells, a cell plate forms to divide the cells.

Key Concepts to Remember:

1. Gene Expression involves DNA → RNA → Protein.

2. Transcription creates mRNA from DNA.

3. Translation uses mRNA to build proteins with the help of tRNA and ribosomes.

4. Cell Division is essential for growth, repair, and reproduction.

• Mitosis creates two identical cells, while meiosis creates four unique gametes.

5. DNA stores genetic information and can be mutated, replicated, and expressed to create proteins.